build: use 3pa for actions/checkout

https://github.com/Brightspace/third-party-actions/blob/master/README.md#why

.github/workflows/foo.yml

@@ -7,7 +7,7 @@ jobs:
         run: ls -alFR || true
 
       # Fails to git clean the foo folder, tries to rm -rf the checkout and then fails
-      - uses: actions/checkout@v2
+      - uses: Brightspace/third-party-actions@actions/checkout
 
       - name: Run a container that outputs to foo/output-file and puts nasty permissions on foo
         uses: ./